embedded3 S2-3b: the flag driver's span knob (full width = the 2D geometry extruded)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/tests/embedded3_flag_wake.rs

@@ -95,13 +95,23 @@ fn flag_2d(x: f64, y: f64, t: f64) -> (f64, f64) {
     (best - FLAG_HALF, v_best)
 }
 
+/// The flag's span: `RTX_E3_FLAG_SPAN` (default 0.2); at the channel's
+/// full width the flag is the 2D geometry extruded (S2-3b).
+fn flag_span() -> f64 {
+    env_f("RTX_E3_FLAG_SPAN", FLAG_SPAN)
+}
+
 /// The flag in 3D: the extruded capsule cut to the span with edges
-/// rounded to radius `r`.
+/// rounded to radius `r` (no cut at the full width).
 fn flag_3d(x: f64, y: f64, z: f64, t: f64, r: f64) -> (f64, f64) {
     let (d2, v) = flag_2d(x, y, t);
+    let span = flag_span();
+    if span >= H {
+        return (d2, v);
+    }
     let zc = 0.5 * H;
     let q1 = d2 + r;
-    let q2 = (z - zc).abs() - 0.5 * FLAG_SPAN + r;
+    let q2 = (z - zc).abs() - 0.5 * span + r;
     let outside = (q1.max(0.0).powi(2) + q2.max(0.0).powi(2)).sqrt();
     (outside + q1.max(q2).min(0.0) - r, v)
 }
@@ -174,7 +184,8 @@ fn flag_wake_on_the_device() {
     }
     solver.initialize(&mut field);
     println!(
-        "  flag wake ny {ny}: {nx}×{ny}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
+        "  flag wake ny {ny} (span {}): {nx}×{ny}×{nz} = {} cells, h {h:.4e}, dt {dt:.3e}, {periods} periods = {t_end:.3} s, {} steps",
+        flag_span(),
         g.cells(),
         (t_end / dt).ceil() as usize
     );
@@ -256,8 +267,9 @@ fn flag_wake_on_the_device() {
                 }
             }
             if phase_due {
+                let tag = if flag_span() >= H { "full" } else { "free" };
                 let path = std::path::Path::new(vtk_dir.as_ref().unwrap())
-                    .join(format!("flag_ny{ny}_phase{next_phase:02}.vtk"));
+                    .join(format!("flag_{tag}_ny{ny}_phase{next_phase:02}.vtk"));
                 write_vtk(&path, &field, Some(mask)).expect("vtk");
                 next_phase += 1;
             }